Reinforced thermoplastic and thermoset materials have wide application in, for example, the aerospace, automotive, industrial/chemical, and sporting goods industries. Thermosetting resins are impregnated into the reinforcing material before curing, while the resinous materials are low in viscosity. Thermoplastic compositions are more difficult to impregnate into the reinforcing material because of comparatively higher viscosities. On the other hand, thermoplastic compositions offer a number of benefits over thermosetting compositions. For example, thermoplastic prepregs are easier to fabricate into articles. Another advantage is that thermoplastic articles formed from such prepregs may be recycled. In addition, a wide variety of properties may be achieved by proper selection of the thermoplastic matrix.
Fiber-reinforced plastic materials are usually manufactured by first impregnating the fiber reinforcement with resin to form a prepreg, then consolidating two or more prepregs into a laminate, optionally with additional forming steps. Consolidation is typically necessary to remove voids that result from the inability of the resin to fully displace air from the fiber bundle, tow, or roving during the processes that have been used to impregnate the fibers with resin. The individually impregnated roving yarns, tows, plies, or layers of prepregs are usually consolidated by heat and pressure, or with heat and vacuum as by vacuum-bag molding and compacting in an autoclave. The consolidation step has generally required the application of very high pressures or vacuums at high temperatures and for relatively long times.
In the past, a thermoplastic composition has typically been heated, slurried, commingled, or diluted with solvents in order to reduce the viscosity of the composition before it is used to impregnate the reinforcing material. These methods have suffered from serious drawbacks. In the case of using solvent to reduce viscosity, the solvent must be driven off after the impregnation step, resulting in an additional step in the process as well as unwanted emissions. Moreover, the desired matrix may be insoluble in common solvents. In the case of heating the thermoplastic matrix in order to reduce its viscosity, the dwell time of the resin in the heated zone may result in degradation of the resin with attendant decrease in the desired mechanical properties. Furthermore, the molecular weight of the resin may need to be kept lower than would be desired for properties of the ultimate product in order to facilitate the impregnation step. Finally, as noted above, known processes for impregnating thermoplastic resin into reinforcing materials have required lengthy consolidation of the prepreg materials at high temperatures and pressures in order to develop the best physical strength and other properties and to minimize or eliminate outgassing during the consolidation or in later steps, e.g., finishing processes. Outgassing during consolidation results in voids within the composite that can cause microcracking or premature delamination that may adversely affect mechanical properties; outgassing during coating steps tends to cause pinholing or popping in the substrate or coating, resulting in an undesirably rough and blemished surfaces or finishes.
Cochran et al., U.S. Pat. No. 5,236,646, disclose that a process using vacuum of up to about 28 inches of mercury below atmospheric pressure and temperatures above the melting point of the resin requires a shorter time for consolidation as compared to a process that uses high consolidation pressures of from about 100 to 300 psi. However, the consolidation step still requires a dwell time under vacuum of up to sixty minutes or more.
Because the length of time typically required to properly consolidate the prepreg plies determines the production rate for the part, it would be desirable to achieve the best consolidation in the shortest amount of time. Moreover, lower consolidation pressures or temperatures and shorter times will result in a less expensive production process, for instance due to lowered consumption of energy per piece for molding.
The present invention provides a new process for preparing prepregs, novel prepregs, and articles of reinforced materials that offers significant advantages over the processes described above. In the methods according the present invention, the reinforcing material is heated before being impregnated with the resinous matrix composition. The impregnated roving or tow that is produced according to the present inventive process has substantially no voids and can therefore be quickly and easily formed into a desired article having no voids or essentially no voids without the lengthy consolidation processes necessary for prepregs formed by other processes. In other words, the roving bundle is fully, or substantially fully, wet out. The only process that must take place in forming an article is fusion between impregnated bundles, and it is possible to use temperatures, pressures, and/or times during such forming operations that are significantly reduced over prior art processes.
The present invention also provides a method of making a molded article using the prepreg of the present invention.